Wireless communication systems are known. One known system is illustrated schematically in FIG. 1. The area covered by a wireless communication network 2 is divided into a number of cells 4. The cells may be side by side and/or overlapping. Each cell 4 is provided with a base station 6. Each base station 6 is arranged to communicate with mobile stations 8 or other user equipment located in the cells.
A number of different standards are known which regulate the communication between the base stations and the mobile stations. One commonly used standard is the GSM (Global System for Mobile Communications) standard. This is a digital communication system. In GSM, data is transferred between the mobile stations 8 and the base stations 6 as a radio signal over a physical channel which may use frequency and/or time division multiplexing to create a sequence of radio frequency channels and time slots. Each frequency band is divided into time division multiple access frames, with 8 users per frame. Each user is allocated time to send a single burst of information. Typically, the mobile station and base station which are in communication will use different frequency bands.
GSM can, in some implementations, use GMSK (Gaussian Minimum Shift Keying) modulation. GMSK modulation uses the phase of the radio signal in order to transmit the data. The phase of the signal is of course dependent on the frequency of the signal. In order to correctly identify the transmitted data, the frequency of the signal received at the receiving one of the base station and the mobile station should be within defined limits as compared to the intended frequency of transmission of that signal. If the frequency has shifted beyond these limits, then errors in the recovery of the data may occur.
Errors in the frequency at the receiving one of the mobile station and base station can occur for a number of reasons. For example, this can occur if one of the mobile station and the base station is moving. Usually, of course, the mobile station will move. Changes in the frequency can of course occur due to the Doppler shift. This effect is particularly marked when the mobile station is moving relatively fast. For example, high speed trains having a speed of around 330 km/hour are being proposed. At those speeds, the Doppler shift introduced by the movement of the mobile station would result in a relatively large frequency change. It should of course also be noted that movement at slower speeds will also result in Doppler shifts.
Movement of the mobile station relative the base station is not the only source of frequency changes. Other errors may be introduced. For example, multi-path propagation may change the frequency of the signal received. The oscillator of the transmitter may not be working correctly, for example due to changes in temperature, and accordingly the transmitted signal and hence the received signal are not at the correct frequency. Additionally, adverse weather conditions particularly very hot or very cold weather can change the condition of the radio channel which results again in a frequency shift of the received radio signals. In general, the changes in frequency are introduced either by radio frequency impairments or change in channel characteristics. The radio frequency impairments include multi-path propagation, and variation in the crystal oscillator characteristics. The change in channel characteristics include the effects due to movement and changes in weather conditions.
Generally, the GSM standard is reasonably robust. As such, it is able to cope with some variation in the frequency. However, errors from more than one source may be present which cumulatively provide a relatively large frequency error.
Additionally, very fast moving mobile stations can introduce a relatively large frequency shift on their own.